1. Field of the Invention
The present invention relates to: a liquid composition suitable for making droplets from a liquid containing at least one of proteins and peptides and a method of ejecting such droplets.
2. Description of the Related Art
Currently, many attempts have been conducted to utilize a protein solution as liquid droplets. For example, for the drug delivery method, the liquid droplets have been considered to be applied in transmucosal administration because of, for example, advantages in that only a small amount of protein may be required in the production of a biochip or biosensor and the protein may be integrated easily. In addition, attentions have been paid on a method of using a fine liquid droplet of protein for control on crystallization of protein and also for screening of a physiologically active substance (see, for example, Japanese Patent Application Laid-Open No. 2002-355025, Allain L R et al., “Fresenius J. Anal. Chem”, vol. 371, p. 146-150, 2001, and Howard E I and Cachau R E, “Biotechniques”, vol. 33, p. 1302-1306, 2002).
In recent years, mass production of proteins, particularly useful proteins such as enzymes and those having physiological activities has become possible by any technology such as genetic recombinant technology. Therefore, the process of making protein into liquid droplets can be effective means in the field of searching, utilizing, and applying a novel protein medicine. More specifically, there are increasing significant demands on means for providing patients with many pharmaceutical agents by microdroplets. In particular, microdroplets have become important for the administration of proteins, peptides, and other biological materials from the lungs. In other words, the lung administration have been remarked as an administration route in place of an injection of a macromolecule peptide-based drug represented by insulin because the lungs have lung alveolis with their own extensive surface areas of 50 to 140 m2 and the epithelium provided as a barrier of absorption is as thin as 0.1 μm, while the enzyme activities of the lungs are smaller than those of the gastrointestinal tract.
In general, the deposition of microdroplets of drug in the lungs has been known to depend largely on the mass median aerodynamic diameters thereof. In particular, the delivery of the microdroplets to the lung alveolis in the deep portions of the lungs essentially requires the administration of the droplets with high reproducibility which present between 1-5 μm of particle size and have a narrow particle size distribution.
As a method of preparing uniform droplets with a narrow particle size distribution, the use of a suitable droplet generator diverted from those used in inkjet printing in the production of extremely fine droplets and the application of the droplets have been reported in the art (see, for example, U.S. Pat. No. 5,894,841 and Japanese Patent Application Laid-Open No. 2002-248171). Here, the specific inkjet printing method concerned involves leading liquid to be ejected into a small chamber where the liquid is subjected to physical power, thereby allowing droplets of the liquid to be ejected from orifices. A discharging method may be any one of those known in the art, such as one that generates air bubbles spouting droplets through orifices formed on a chamber by means of electrical and thermal transducers such as thin-film resistors (i.e., a thermal inkjet method) and one that ejects liquid directly from orifices formed on a chamber by means of piezoelectric transducers (i.e., a piezo inkjet method).
For allowing the lungs to absorb a drug, the dose of the drug should be controlled. Therefore, making droplets from the liquid by the inkjet method, which is capable of adjusting the ejection amount thereof, is a very preferable configuration. On the other hand, however, the ejection of a solution should be surely carried out in this case. However, the ejection of a protein solution is unstable when the protein solution is only controlled with respect to its surface tension and coefficient of viscosity. Therefore, the difficulty in quantitative ejection has often occurred.
Problems associated with the process of making droplets of a protein or peptide using the inkjet method are due to the fact that the tertiary structure of the protein is brittle. Thus, the aggregation or degradation of the protein may occur when the configuration thereof is destroyed. When the process of making droplets is carried out using the inkjet method, the structures of many proteins may become unstable owing to physical stress such as pressure or shearing stress or to high surface energy peculiar to microdroplets (if the thermal inkjet method is employed, heat is further applied in addition to the above stress). In particular, in the case of the inkjet method, those physical actions are extremely larger than the shearing force and thermal energy to be applied by conventional stirring or heat treatment (for example, in the case of the thermal inkjet method, approximately 300° C. and 90 atm are applied momentarily). Furthermore, several physical stresses are impressed at the same time, so that the stability of protein may tend to be substantially lowered, compared with the case of usually handling the protein. On this account, the conventional technology for stabilizing protein may be insufficient when the inkjet technology is employed. If such a problem occurs, the protein may be aggregated at the time of making droplets and nozzles may be then clogged, thereby making it difficult to ejection droplets.
Furthermore, droplets having diameters of 1 to 5 μm, which are suitable for the lung administration, are extremely smaller than those having diameters of approximately 16 μm used in any printer commercially available at present. Therefore, a larger surface energy or shearing stress may be impressed on the droplets than on the droplets used in the printer. Therefore, it is more difficult to eject microdroplets suitable for inhaling protein into the lungs.
Meanwhile, a method in which a surfactant (surface-active agent), a water-soluble high polymer such as glycerol, a saccharide of various types or polyethylene glycol, and albumin or the like, are added is known as a method by which proteins and peptides are stabilized. However, in may cases, this method is little effective, or not effective at all, in improving the stability of ejection when proteins and peptides are ejected by the thermal inkjet method.
In the pamphlet of International Publication No. WO 02/094342, there is disclosed a compound for regulating surface tension and a method of adding a humectant for a liquid composition for the pulmonary absorption of a droplet prepared using a thermal inkjet method. Here, for increasing the stability of protein in a solution provided as droplets by the surface tension, viscosity, or moisturizing action of the solution, a surfactant or a water-soluble polymer such as polyethylene glycol is added.
However, there is no description about the stability of discharging, and also the addition of a surfactant or a water-soluble polymer exerts insufficient effects when the concentration of protein or peptide increases. In addition, there are more surfactants which have been recognized as of no effect at all than those having effects. Besides, the stability is defined by not only the surface tension and viscosity. Therefore, the method disclosed in the document is not a common practice for the stabilization of ejection. Therefore, for actual use, any liquid to be used for an ejection purpose, which is capable of discharging protein or peptide in a stable manner, becomes essential.